Energy and Chemical Change - 7

Endothermic reactions absorb heat and thus...

decrease the temperature of the reaction mixture.

Energy diagrams We can represent the changes in a chemical reaction with an ___.

energy profile.

The temperature of the system will be observed to rise in an ___. In an exothermic reaction, the ____ of the reaction mixture will rise.

exothermic change temperature

ΔH is negative for ___ and ΔH is positive for __. The unit of ∆H is kJ·mol⁻¹

exothermic reactions endothermic reactions

If two atoms ___ energy in an attempt to form a bond, then the bond will not form. Explain why.

gain They need the benefit derived from a reduction of energy, not a gain of energy.

The activation energy

he minimum energy required to start a chemical reaction OR the energy required to form the activated complex

Reactions can only happen when the reactant particles collide, but...

most collisions are not successful in forming product molecules.

If the particles collide with less energy than the activation energy...

nothing important happens. They bounce apart. You can think of the activation energy as a barrier to the reaction.

Exothermic reactions

reactions which transfer potential energy into thermal energy.

Endothermic reactions

reactions which transfer thermal energy into potential energy.

A stronger bond ___ energy to break and ____ energy when formed.

requires more releases more

When chemical reactions occur, as well as the formation of the products, there is also an overall energy change which can often be detected as a... What does this mean?

temperature change. This means the products have a different amount of energy than the original reactants.

The bond energy

the amount of energy released when a bond forms.

If two atoms can release energy by forming a bond, then...

the atoms will be more stable by staying together than they would be as individual atoms. As a result, the atoms remain in the bonded condition.

Energy in chemical bonds Chemical reactions are always associated with...

the breaking of existing bonds (in the reactant) and the formation of new bonds (to form the product).

Any chemical reaction results in ___ and ___.

the breaking of some bonds (needing energy) and the making of new ones (releasing energy). Obviously some bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds.

The difference between ___ gives the overall energy change for the reaction. This heat change is called the ___ is denoted by delta H, ΔH.

the energy levels of the reactants and products enthalpy change

When an activated complex is formed during a reaction, this complex can lead to...

the formation of new bonds OR reformation of the old bonds (reversibility of reaction).

Heat of reaction (∆H)

the net change of chemical potential energy of a system.

Collisions only result in a reaction if... This minimum energy required is called the ____ for the reaction.

the particles collide with enough energy to get the reaction started. activation energy

∆H endothermic ∆H = +131 kJ·mol⁻¹ What does this mean?

(A value of +131 kJ·mol-1 means that 131 kJ is absorbed for every mole of product formed.)

The ____ process always releases energy.

bond forming

The ____ of a bond always requires energy.

breaking

exothermic reaction

- When the products contain less energy than the reactants, - heat is released or given out to the surroundings

endothermic reaction

- When the products have more energy than the reactants, - heat is taken in or absorbed from the surroundings

Examples of exothermic reactions

- the burning or combustion of hydrocarbon fuels - the burning of magnesium, reaction of magnesium with acids - the neutralisation of acids and bases;

Examples of endothermic reactions

- the thermal decomposition of limestone - the cracking of oil fractions.

If the change can take place spontaneously...

...the temperature of the reacting system will drop but it is more likely that the reactants must be heated to speed up the reaction and provide the absorbed heat.

∆H exothermic ∆H = -393 kJ·mol⁻³ A value of -393 kJ·mol₋¹ means that

393kJ is released for every mole of product formed.)

For the following reaction: AB + C → A + BC the formation of an activated complex as a transitional step can be represented as:

AB + C -> [ABC] -> A + BC where [ABC] is the activated complex

Equation for endothermic reaction

Example: C(s) + H₂O(g) → H2 (g) + CO(g) ∆H = +131 kJ·mol⁻¹

Equation for exothermic reactions

Example: C(s) + O₂ (g) → CO2 (g) ∆H = -393 kJ·mol⁻³

Explain bond energy and what it corresponds to.

It is a direct measure of the amount of stability gained when two atoms establish a chemical bond. Inversely, bond energy corresponds to the amount of energy that is required to break a bond.

Which collisions result in a reaction?

Only those collisions which have energies equal to or greater than the activation energy result in a reaction.

___ have generally have low activation energy which, once started, are self-propagating.

Spontaneous reactions

Relationship between - The amount of energy that is required to break a bond - and the amount of energy released when the bond is formed.

The amount of energy that is required to break a bond is exactly equal to the amount of energy released when the bond formed.

Consider the reaction between hydrogen gas and oxygen gas to produce water and explain. 2H₂(g)+ O₂(g) → 2H₂O(g)

The bonds between the hydrogen atoms and the oxygen atoms need to be broken before a new bond can form between the oxygen atom and two hydrogen atoms.

enthalpy

The heat content of a chemical system

If the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. You can think of the activation energy as...

a barrier to the reaction.

The activated complex

a high energy, unstable, temporary transition state between the reactants and products

When activation energy is added to the reactants, a so-called ___ is formed.

activated complex

Energy is released when we replace... Explain why.

unstable bonds with more stable bonds. This occurs because matter always acts to go to the most stable state possible.